I am afraid I am not qualified to be the tie-breaker here. It is so brilliant that you have had your lovely dog from the rescue treated and we are glad to hear no adverse reaction to date. It is going to have to be the vet who administered the immiticide who advises you. Although heartworm preventatives will not kill the adult worms and only the immiticide will do that, you are rightly concerned about any possible re-infection during the period of treatment. Your vet will be able to give you the definitive answer and put your mind at rest.
We would love to hear from you and maybe publish your dog’s story, do let us know how you get on.
All the best
Dermatologic: alopecia, urticaria, skin rashes, toxic epidermal necrolysis, erythema multiforme, erythema nodosum, fixed drug eruption, lichen planus, pustular reaction, systemic lupus erythematoses, bullous reactions, including Stevens-Johnson syndrome, photosensitive dermatitis, photosensitivity reactions, including rare cases resembling porphyria cutanea tarda (pseudoporphyria) or epidermolysis bullosa. If skin fragility, blistering or other symptoms suggestive of pseudoporphyria occur, treatment should be discontinued and the patient monitored.